The present invention relates to a demodulation system for a playback device in a video system employing a carrier frequency signal containing useful information in the zero passages of the signal, the demodulation system being of the type including a limiter which limits the signal fed to it in a manner which is not completely symmetrical and a demodulator coupled to the output of the limiter.
When a record carrier is scanned by a scanning device to play back video signals, the scanning may occasionally cause the carrier frequency signal, which can be, for example, frequency modulated with the video signals, to be temporarily partially lost, i.e. to experience a "break in", where the amplitude of the useful portion of the signal is temporarily strongly reduced. If such a break in the useful portion of the signal occurs, the electron beam of the picture tube connected to the scanning device will not be properly modulated during this break-in.
This signal break-in will cause a dot or stripe to be produced on the screen of the picture tube which is excessively bright or dark compared to the other sections of the picture on the screen. Such phenomena are extremely annoying for the viewer. Consequently, many efforts have been made to reduce the effect of such signal break-ins to a minimum in order to provide a better television picture.
In one known arrangement for avoiding the effect of such break-ins, the carrier frequency useful signal is divided into two practically identical components, one of these components being delayed with respect to the other component by a time interval which is practically equal to the time required by the electron beam in the picture tube for one horizontal scanning period. Under normal conditions, the undelayed component is used and the delayed component is suppressed. If, however, a device for detecting an error in the signal, e.g. a break-in, actuates a switching device, the undelayed component is suppressed and the delayed component is switched on and is then used during the occurrence of the break-in state. While the above-described arrangement does remove the undesirable effect of the break-ins on the useful signal, such an arrangement, however, requires complicated circuitry which, when used in playback devices, substantially increases the cost of such devices.
In view of the fact that break-ins of the useful signal are a relatively rare occurrence in the presently known playback devices and their time duration is very short, such arrangements which require, in addition to the delay elements for storing the video signal, additional circuitry for detecting the break-ins so that switching from the undelayed to the delayed video signal can be initiated, are relatively complicated and only suitable for playback devices which are intended to meet very high viewer demands.
Since the use of recording and playback video devices as home instruments is increasing, these devices must be able to be fabricated by use of mass production techniques while maintaining a satisfactory level of quality for the device and producing a device which operates in a relatively simple manner.
It was previously the practice, in ultrahigh frequency (UHF) radio receivers with frequency demodulators, to connect, ahead of the demodulator, a limiter having a bandpass filter in the anode circuit of the tube. This limiter, however, was used in a transmission path with a relatively small ratio (about 1/100) of the absolute bandwidth to the maximum frequency and with a relatively interference free useful signal in which signal break-ins rarely occur. Consequently, there was no reason in these known arrangements to eliminate interferences caused by signal break-ins. The resonant circuit in the anode circuit in such arrangements was utilized to serve an entirely different purpose from that to which the present invention relates; such resonant circuits were intended to attentuate the harmonics resulting from the limiting process. In addition, this resonant circuit attentuated low frequency noise which is always present.